Conventionally, a lamp device has been used which uses a GX53-type cap portion standardized by the IEC (International Electrotechnical Commission). The lamp device has a flat lamp device body, the GX53-type cap portion is provided on an upper face side of the lamp device body, a flat light source using a fluorescent lamp, LED or the like is arranged on a lower face side of the lamp device body, and a lighting circuit for lighting the light source is housed inside the lamp device body. On the cap portion, a pair of lamp pins each having a large diameter portion at its top end is projected. The lamp pins of the lamp device are inserted and hooked into a socket device by turning the lamp device, the lamp device is held by the socket device, and power is supplied from the socket device to the lamp pins.